This may not fail the metaphor test. Take a real folder in your hands and try to stuff in it a drawer which is too small. Your corner tab will stay in the cor

This may not fail the metaphor test. Take a real folder in your hands and try to stuff in it a drawer which is too small. Your corner tab will stay in the corner while the rest gets "smushed". If anything, this makes it a bit more intuitive for the user as it avoids the confusion of losing controls because of an errant mouse drag.